A terahertz electromagnetic wave is an electromagnetic wave having a frequency from 0.1 to 10 THz (wavelength from 30 to 3000 μm). This wavelength is substantially the same as a range from the wavelength of a far-infrared wave to that of a millimeter wave. The terahertz electromagnetic wave exists in a frequency range between the frequency of “light” and that of a “millimeter wave.” Thus, the terahertz electromagnetic wave has both an ability to identify an object with a spatial resolution as high as that of light and an ability comparable to that of a millimeter wave to pass through a substance. An electromagnetic wave in the terahertz wave band has not been explored so far. Meanwhile, application for example to characterization of a material has been examined that is to be achieved by time-domain spectroscopy, imaging, and tomography utilizing the characteristics of the electromagnetic wave in this frequency band. The terahertz electromagnetic wave has both the performance of passing through a substance and straightness. Thus, using the terahertz electromagnetic wave instead of an X-ray allows safe and innovative imaging or ultrahigh-speed radio communication of some hundreds of Gbps.
Use of a wire grid mainly for a purpose such as polarizing or analyzing of a terahertz electromagnetic wave has conventionally been suggested. Researches have been advanced to achieve such a wire grid.
According to one example of a conventional free-standing wire grid, the wire grid is formed by aligning metal thin lines of a diameter from about 5 to about 50 μm one by one in a parallel fashion at a prescribed interval and affixing the metal thin lines with an adhesive to a meal frame. This free-standing wire grid encounters a limit on an applicable frequency. The free-standing wire grid, applicable as a polarizer for a terahertz electromagnetic wave of about 1.5 THz or more, is generally required to have a fine structure, which is difficult to achieve.
Patent literature 1 discloses a metal plate for a wire grid applicable as a polarizer for a terahertz wave band. FIG. 81 is a perspective view showing the structure of a metal plate 101 for a wire grid disclosed in this literature. FIG. 82 is a plan view showing a part of the metal plate 101 for a wire grid in an enlarged manner. FIG. 83A is a plan view showing a part of FIG. 82 in a further enlarged manner. FIG. 83B is a sectional view taken along cutting line A-A of FIG. 83A.
The metal plate 101 for a wire grid is made of nickel and has a circular plate shape of a diameter from about 20 to about 100 mm, for example. As shown in the drawings from FIG. 81 to FIGS. 83A and 83B, the metal plate 101 includes a plurality of vertical bridge parts 111 extending in the vertical direction in a bridge pattern (thin-line pattern) and at least one cross bridge part 112 substantially orthogonal to each vertical bridge part 111. The vertical bridge parts 111 and the cross bridge part 112 each have opposite ends connected to a flange part 113 of a circular or rectangular shape.
The width of the vertical bridge parts 111 (wire width) and the interval between the vertical bridge parts 111 are parameters that determine the performance of the metal plate 101 for a wire grid and are defined according to the frequency of light to be applied. The metal plate 101 for a wire grid may have a structure applicable to a terahertz electromagnetic wave of 1.5 THz or more and the vertical bridge parts 111 may have a width Wa that can be from 1.5 to 50 μm.
In the metal plate 101 for a wire grid, the cross bridge part 112 has a width at least not falling below a given width and not falling below the width of the vertical bridge parts 111. This allows manufacture of the vertical bridge parts 111 of a thin-line structure having the width Wa from 1.5 to 50 μm. The metal plate 101 for a wire grid has a thickness that should be determined in consideration of physical strength against separation from a substrate, for example, or degradation of the characteristics of transmitted light. This thickness is set at 10 μm.
The width Wa of the vertical bridge parts 111 is determined uniquely as a parameter that determines the performance of the metal plate 101 for a wire grid. A parameter about the cross bridge part 112 such as a width Wb of the cross bridge part 112 or an interval between the cross bridge parts 112 (the number of the cross bridge parts 112) is determined mainly in light of assuring the strength of the metal plate 101 for a wire grid. Thus, the width Wb of the cross bridge part 112 is set not to fall below the width of the vertical bridge parts 111. More specifically, the width Wa of the vertical bridge parts 111 is set in a range from 1.5 to 50 μm. The width of the cross bridge part 112 is set at 15 μm or more to be larger than that of the vertical bridge parts 111.
FIG. 84 shows characteristics obtained by using the metal plate 101 for a wire grid where the width Wa of the vertical bridge parts 111 is 20 μm, the interval between the vertical bridge parts 111 is 60 μm, the width Wb of the cross bridge part 112 is 20 μm, the interval between the cross bridge parts 112 is 5 mm, and the thickness of the metal plate 101 is 50 μm. As understood from a characteristic line α2 corresponding to transmissive arrangement and a characteristic line β2 corresponding to blocking arrangement shown in FIG. 84, the metal plate 101 operates as a polarizer for terahertz light (synonymous with a terahertz wave) at a frequency from 0.1 to 1.5 THz. In this case, if an amplitude direction of the electric field of the terahertz light is orthogonal to the vertical direction in which the vertical bridge parts 111 extend, the transmissive arrangement is produced. If the amplitude direction of the electric field of the terahertz light agrees with the vertical direction in which the vertical bridge parts 111 extend, the blocking arrangement is produced.
According to the description of non-patent literature 1, a wire grid in a terahertz band is formed by using metal pieces of a width of 100 μm and aligned with a pitch of 200 μm. A blocking rate is measured with the thickness of the metal pieces changed between 0.05 mm, 0.1 mm, 0.2 mm, and 0.5 mm. A highest blocking rate is obtained with the largest thickness of the metal pieces, which is 0.5 mm. Transmittance obtained with this blocking rate is found to be about 0.01%.